1. Field of the Invention
The invention lies in the disciplines of electronics, electromagnetic theory, and solid state physics. More specifically, the invention lies in the realm of photoconductive electronic switch devices, and in particular, discloses a preferred geometry or physical orientation of a photoconductive semiconductor medium, GaAs, coupled between two metallic contacts in a manner to obtain maximum electronic field concentration therebetween for maximum electron-hole generation and current flow in the semiconductor bulk area and away from multiple dielectric junction areas to yield a high power, fast, and low rise time switch of increased sensitivity.
2. Description Of Related Art
Until recently, pulser circuit generation of kilovolt level pulses with rise times in the 50 to 200 picosecond range and trigger time jitter in the range of less than 20 picoseconds was virtually impossible to accomplish. With the advent of avalanche diodes and transistors, pulser circuits operating in the above range became a reality; however, such pulser circuits have been somewhat complicated and unwieldy to make and use.
A laser triggered DC biased or pulse biased GaAs, optically active semiconductor, switch would be much easier to use, less expensive to make, and would be capable of achieving a much higher voltage level. By implementing such photoconductive switches, less expensive, more reliable, and longer lived pulsers could be achieved.
Photo conductive switches generally use some optically reactive medium disposed between two electrodes. The medium conventionally operates as a resistive barrier to current flow between the two electrodes until illuminated by light. Upon illumination by light, outer electron energy levels of atoms in the medium absorb a sufficient quantum of photon energy to permit orbiting electrons to be sufficiently energized to break free of their respective atoms creating free and independent negatively charged electrons and leaving behind ionized, positively charged atoms or ions. These positively charged ions, referred to as holes in the case of a solid semiconductor, and negatively charged free electrons, in the presence of an electric field, will cause a current to flow with the flow of electrons and holes thereby changing the medium from a good resistor to a good conductor. Briefly, in a photoconductive switch, light generates carriers in the photoreactive medium that effectively changes the medium from an area of high resistance to an area of low resistance. In making such a conduction change quickly, the medium functions like an electrical switch.
Certain photoconductive switches have been implemented in the past. One such device, generally referred to as a spark gap switch, consists of a pair of electrodes disposed in a medium or environment of inert gas such as Nitrogen. Nitrogen, of course, normally acts as an insulator between the electrodes; however, a few seed carriers normally exist in any gaseous state and with light illumination and concomitant photon energy imparted to the gas, their numbers are increased in sufficient quantity to enable generation of a spark across the contacts. When a spark is discharged across the contacts and through the medium, energy in the electrical discharge is imparted to the Nitrogen atoms and molecules causing further and substantial ionization of Nitrogen atoms yielding positive ions and freeing valence level negative electrons. With free mobility of charged particles, the insulator medium of high resistance changes to a conductor medium of low resistance, and in the presence of an electrical field conduction occurs in the manner of a photoconductive switch.
A spark gap photo conductor switch can switch a great deal of current and can react very fast with respect to a rapid pulse rise time, but it carries with it inherent timing error uncertainties with respect to jitter. Jitter is the time between light illumination and the spark gap generation, i.e. the moment of switching and current flow. Jitter in a spark gap switch can be substantial, and therefore, the exactness with which the photoelectric switch is to operate, i.e. to turn on and turn off, can vary a great deal.
Another prior art optically reactive electrical switch is a thyristor. A thyristor is a solid state type switch consisting of multiple layers of PNPN type junctions. Illumination of the PN junction imparts energy to the P and N doped semiconductor layers thereby generating more free electrons and leaving behind positive holes. A PN junction by its nature normally maintains seed carriers and a charge barrier with the alignment of positive holes on the P side and negative electrons on the N side of each PN junction. With the creation of excess electrons and holes by illumination, an overflow of charge occurs yielding a breakdown of each respective and successive charge barrier. The feedback of barrier breakdown to preceding junctions induces creation of even more charge carriers which rapidly creates an avalanche of charge carriers and effectively converts a nonconductive medium into a conductive medium with the flow of electrons and holes.
Photo conductive switch speed is dependent in large part on the existence of seed carriers and the growth of electron-hole pairs, and in a semiconductor, the switch process starts with thousands of electron hole pairs as seed carriers. As a result the thyristor solid state, photoconductive switch does have the advantage of faster switch reaction and decreased jitter, but it also has increased pulse rise time because carriers must travel from junction to junction for the required feedback before other carriers can inject in and multiply to cause the avalanche which changes the medium from a resistive element to a conductive element.
Other variations of solid state photoconductive devices exist in the art such as a photoconductive detector disclosed in Ashley, U.S. Pat. No. 4,926,228, an optical detector disclosed in Okabe, U.S. Pat. No. 4,839,510, and a high speed optoelectric switch disclosed in Leonberger, U.S. Pat. No. 4,376,285.
Ashley discloses an electrode embedded in an optically reactive semiconductor to accumulate carriers in the vicinity of an output; Okabe discloses a photoconductive contact geometry for carrying higher current; and Leonberger discloses electrodes disposed on the surface of a photoreactive semiconductor operating as a switch.
Although these devices utilize similar theoretical applications as the invention disclosed herein, the embodiment, application, and operation of each are such that they neither anticipate nor suggest the present invention. Indeed, none of the prior art has addressed the long standing problems encountered in the need to conduct and accurately switch a high current with low power illumination. In particular, there exists a need for higher concentration of electric lines of force in the bulk, optically active and illuminated portion of the semiconductor for greater electron-hole generation capability which inherently increases the optical switch sensitivity and thereby diminishes the amount illumination needed to activate the switch. In addition, there exists a need to attenuate lines of force at the semiconductor surface and at triple junctions of various media of different dielectric constants to diminish electron hole formation which thereby diminishes the tendency for electrical shorting or arcing at a triple dielectric junction.
The invention disclosed herein is a photoconductive switch that describes a unique geometry of a photoreactive semiconductor disposed between two metallic electrodes in such manner to increase the switch efficiency by concentrating the electric field through a smaller volume and area of the semiconductor than heretofore has been possible. In particular, the invention relates to a method and apparatus for diminishing the number of electric lines of force at the dielectric surface and at the triple junction where the dielectric constant of the semiconductor is caused to mesh with the dielectric constant of the metal contact and the dielectric constant of a third medium such as air or epoxy. Concentrating the electric field in the bulk area of the semiconductor and away from the triple junction is needed to increase switch sensitivity and in certain applications that require extremely high electric fields in the active bulkswitch area and to limit fields and related carrier injection in the triple junction area.
Laser diode triggered Gallium Arsenide (GaAs) photoconductive switches require extremely high electric fields in the active bulk switch area. With a proper GaAs, metal electrode, and dielectric design, the high bulk electric fields can be achieved without carrier injection at the triple junction and any residual carriers injected at the triple junction can easily be avalanche multiplied in the high electric field in the bulk material. Such carrier generation and avalanche multiplication usually prevents the achievement of suitable high enough electric field for DC biased switch devices. This invention indicates a design that prevents or eliminates carrier injection at the triple junction, and eliminates injected carrier multiplication by reducing the electric field along their travel path. The invention allows DC biased devices to be fabricated with extremely high bulk electric fields while concomitantly having very low triple junction fields that will not generate carriers.
By such means, pulse rise time and timing jitter are greatly diminished with the added benefit and need for less illumination power consumption. Switch speed is proportionately and desirably increased by faster electron-hole generation in an area of less illumination due to the effect of the concentrated electric field.